update README.dm

README.md

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 Compiled object-oriented programming language
 
 ## Compilation pipeline
-Lexer --> Preprocessor (unimplemented) --> Parser --> Expander --> Validator --> Codegen (--> Optimizer (LLVM's opt))
+Lexer --> Preprocessor (unimplemented) --> Parser --> Expander --> Validator --> Codegen [--> Optimizer (LLVM's opt)]
 
 ## Goals
 - [ ] Self-hosted